In an optical card 1 as shown in FIG. 5, the format of recorded information is such that a main track 3 consists of a multitude of subtrack row 2, information being formed according to the presence or absence of dots thereon as shown in FIG. 6 in an enlarged view.
Several methods have been employed heretofore for reproducing recorded medium having the same information as the optical card 1 in large quantities by means of a mask having a pattern of the main track 3. For example, a first method utilizes a photoresist and an etching, where the recorded medium obtained therein has a construction that a photoresist 6 having a desired information pattern and a light reflecting layer 7 are formed on a substrate 5 and further a protective film 8 is provided thereon, as shown, for example, in FIG. 7. Then, a second method known heretofore is such that a pattern of a glossy silver 9 is formed on a substrate 5 by exposing and developing a silver emulsion in a desired pattern, and a protective film 8 is then formed thereon as shown, for example, in FIG. 8 (Japanese Pat. Laid-Open No. 49296/1981, for example). Further, a third method is that in which a photoresist exposed and developed in a predetermined pattern is plated to form a rugged stamper, and an optical card is reproduced by press method using the stamper.
The first method described above is disadvantageous in that the etching process is time-consuming and the controllability of a large surface card is poor. The second method described above is disadvantageous because silver used therein is expensive. In addition, both the first and second methods above require many steps as well as a wet process. The third method is superior in mass productivity to the first and second methods, but it is still disadvantageous in that information recording density is not raised enough because the contrast is obtained by a phase method where rugged dots are formed, the resulting regenerative output being therefore differential, and because the tolerance of defocus is small.
In view of the foregoing, the present invention provides a method of making optically recorded information medium which permits the reproduction in large quantities and at a reasonable cost as well.